The Radio Data System (RDS) is used to broadcast information together with Frequency Modulated (FM) radio signals for automobile radios as well as home based FM receivers. The FM broadcast signal with the embedded RDS signal is known as a multiplex (MPX) signal. This signal includes information such as program identification including type of program (news, music, etc.), traffic information, title of a song, artist, and the like. In some automotive radios, the radio can switch to another station with the same programming when a given signal deteriorates. The RDS signal may also be accompanied by a Motorist Information System (referred to commonly as ARI) signal. Both the RDS and ARI signals are relatively narrowband signals spaced at 57 KHz (see FIG. 2).
Various problems (interference or anomalies) have been observed in radios using RDS. For example, when a signal rapidly deteriorates the user may be presented with low quality information due to the weak signal conditions. It is important that the RDS minimizes the occurrence of low quality information. In those instances when the FM signal combined with noise results in an amplitude modulated (AM) signal that exceeds 100% modulation, large impulse noise components may be introduced into the RDS signal after FM demodulation and thus interfere with proper demodulation of the RDS signal. Normally the RDS signal is a suppressed carrier signal, however some broadcasters do not observe this convention and broadcast an MPX signal with an unsuppressed subcarrier. This results in another form of interference in attempts to properly demodulate the RDS signal.